Dual circular polarization waveguide system

ABSTRACT

A dual circular polarization waveguide ( 24 ) is described which has a septum ( 36 ) which divides the waveguide ( 24 ) into two separate compartments ( 38, 40 ) each with a probe ( 30 A,  30 B) passing through the end wall ( 32 ) of the waveguide ( 24 ) into the compartment ( 38, 40 ) to detect respective signals in each of the compartments ( 38, 40 ). The septum ( 36 ) is proportioned and dimensioned to convert the left and right circularly polarized signals, into substantially linearly polarized signals as the signals pass along the waveguide ( 24 ) past the septum ( 36 ) so as by the time the signals reach the probes ( 30 A,  30 B) they are linearly polarized. The probes ( 30 A,  30 B) which pass through the rear wall ( 32 ) of the waveguide ( 24 ) are oriented such that they couple into the magnetic field of the primary or fundamental waveguide mode. These probes ( 31 A,  30 B) do not require to be orthogonal to each other but each probe ( 31 A,  30 B ) has a free end disposed in proximity to a waveguide wall or the septum ( 36 ) within a respective compartment ( 38, 40 ) so that the probe ( 30 A,  30 B) is capacitively coupled to the waveguide wall or septum ( 36 ) to allow the probe ( 30 A,  30 B) to couple into the respective magnetic field in the compartment ( 38, 40 ).

The present invention relates to a dual circular polarity probewaveguide system, and to a waveguide for use in such a system, forreceiving circularly polarised signals and for converting the circularlypolarised signals into linearly polarised signals.

In many jurisdictions, such as the United States and South America, thepolarisation system used to transmit satellite signals is known as DualCircular (Left and Right Polarisation), as opposed to Dual Linear as isused in Europe and other parts of the world.

Broadcasting standards to meet certain design criteria in signalreception are becoming more demanding. One example is the current U.S.Standard for isolation performance which requires a 25 dB signalseparation is between left and right hand circular polarised signals.This standard is exceedingly difficult to achieve in a single waveguideover the whole of the required frequency band. In the United States thefrequency band is 12.2-12.7 GHz and the frequency band in South America,Russia and many other countries is 11.7-12.2 GHz. Ideally it isdesirable to manufacture a single waveguide for use in a low noise blockor the like which can be used in all of the countries and satisfies theisolation standards for each of the countries across the whole of theband in these respective countries.

It is known to use a stepped septum polariser to convert circularpolarisation to linear polarisation, as disclosed in a paper by Chen, MH and Tsandoulas, G N (Communications, 1973). When used in a signalreceiving system, the left and right circular polarised signals areseparated into different rectangular waveguides and propagate in theTE10 mode. In the above paper the signals pass through the waveguide forfurther processing and/or detection. There is no disclosure as to howsuch further processing and/or detection is achieved.

U.S. Pat. No. 5,245,353 to Gould discloses a waveguide with dual probesextending through a back wall coaxially into the waveguide. In thisarrangement the probes are oriented such that each probe couples to aprimary waveguide mode but does not couple to a first higher waveguidemode or the TEM mode. This is achieved by arranging the probes so thatthey are orthogonal to each other and to the primary waveguide modessuch as TE11 in the circular waveguide.

A disadvantage of this arrangement is that the orthogonal probes arelocated in a single waveguide and some cross-coupling still occursbetween the probes limiting the isolation between the orthogonallypolarised signals.

U.S. Pat. No. 5,331,332 discloses a rectangular waveguide with a singleprobe launched from the end of the waveguide and a partial transmissionwall extending along the waveguide from the rear wall of the waveguideand surrounding part of the probe. The transmission wall is stated toenhance the transmission of microwave signals therealong and also toallow adjustment of impedance presented to the waveguide by thetransmission walls. This waveguide structure is relatively difficult tomanufacture and there is no disclosure of converting circularlypolarised signals into linearly polarised signals.

An object of the present invention is to provide an improved waveguidewhich obviates or mitigates at least one of the disadvantages ofaforementioned waveguides.

This is achieved by providing a symmetrical waveguide which has a septumwhich divides the waveguide into two separate compartments each with aprobe passing through the end wall of the waveguide into the compartmentto detect respective signals in each of the compartments.

The septum is proportioned and dimensioned to convert the left and rightcircularly polarised signals, into linearly polarised signals as thesignals pass along the waveguide past the septum so that by the time thesignals reach the probes they are linearly polarised. The probes whichpass through the rear wall of the waveguide are oriented such that theycouple into the magnetic field of the primary or fundamental waveguidemode. These probes do not require to be orthogonal to each other buteach probe has a free end disposed in proximity to a waveguide wall orthe septum within a respective compartment so that the probe iscapacitively coupled to the waveguide wall or septum to allow the probeto couple into the respective magnetic field in the compartment.

One of the main advantages of this arrangement is that it providesexcellent isolation between the probes since they are effectivelycontained in different waveguides. This results in a waveguide and LNBwhich provides isolation in excess of the 25 dB specification across thewhole of the 11.7-12.7 GHz band used in the United States, South Americaand other countries.

According to a first aspect of the present invention there is provided adual probe waveguide structure for use in a LNB (low noise block) forreceiving a left (L) and a right (R) circularly polarisedelectromagnetic radiation signal and for converting the circularlypolarised signals into linearly polarised signals, the waveguidestructure comprising:

-   -   a waveguide housing of a substantially symmetrical cross        section, said waveguide housing having a front aperture and a        rear waveguide wall, a septum disposed within the housing and        coupled to the rear waveguide wall and the housing to separate        the waveguide into two waveguide compartments, one compartment        for receiving and converting the left circular polarisation        signal into a first linearly polarised signal and the other        compartment for receiving and converting the right circular        polarised signal into a second linearly polarised signal        orthogonal to the first linearly polarised signal,    -   a first probe extending into said first waveguide compartment        from the rear waveguide wall and a second probe extending into        the second rear waveguide compartment from the second waveguide        wall the first and second probes each having a free end,    -   the probes being oriented and arranged such that the free ends        of the probes are disposed in proximity to the waveguide wall or        septum in each respective compartment such that the probes        capacitively couple into the magnetic field of the primary or        fundamental waveguide mode in the waveguide compartment.

Preferably, the waveguide housing is square in cross-section.Alternatively, the waveguide housing is circular in cross-section.

Preferably also, the septum is stepped. Alternatively, the septum isnon-stepped and has a curved edge.

Preferably, the rear wall of the waveguide is integral with thewaveguide housing. Alternatively, the waveguide wall is provided by aground plane of a circuit board disposed at the end of the waveguide.

Conveniently, two probes are mounted in the circuit board, one probeextending into a respective compartment.

Conveniently, the probes are circular in cross-section. Alternatively,the probes may be of any other suitable cross-section, such as square,rectangular, hexagonal or triangular which maximises the coupling of themagnetic field from the compartment.

Preferably, each of the probes has a first portion which extendssubstantially parallel to the waveguide axis into the respectivewaveguide compartment and a second portion coupled to the first portionat an obtuse angle, each second portion having its free end disposedtowards the septum and the leading end of the other probe.

Preferably the free ends of the probes converge towards each other andtowards the septum. Alternately the free ends of the probes diverge fromthe septum towards the waveguide wall.

Preferably the probes are located in respective compartments such as tobe reflected about the plane of the septum.

Preferably also, the waveguide housing, rear wall and septum, are formedfrom a die-cast metal selected from aluminium, zinc, magnesium or alloysof these elements such as MAZAC, a zinc alloy; LM24 an aluminium alloy,and AZ91D, a magnesium alloy.

Conveniently, the septum is substantially the same thickness from therear wall to the stepped or curved edge of the septum, the septum havinga draft angle about 1° per side to facilitate release of the waveguideafter being die-cast.

According to a further aspect of the present invention, there isprovided a method of converting left and right circularly polarisedsignals into linearly polarised signals comprising the steps of:

-   -   passing a left and right circularly polarised combined signal        into a waveguide housing, separating the left and right        circularly polarised signals within the housing and converting        the circular polarisation of the left and right circular        polarised signals into linearly polarised signals,    -   passing the separated linearly polarised signals into different        waveguide compartments to isolate the linearly polarised signals        from each other, and coupling into the magnetic field of the        primary or fundamental waveguide mode for each signal within        said waveguide compartments.

These and other aspects of the present invention will become apparentfrom the following description, when taken in combination with theaccompanying drawings, in which:

FIG. 1 is a diagrammatic view of a low noise block in accordance with apreferred embodiment of the present invention;

FIG. 2 a is a perspective and partly broken-away view of the waveguideshown in FIG. 1 with a waveguide wall removed;

FIG. 2 b is a similar view to FIG. 2 a, from a different angle, with thewaveguide walls removed;

FIG. 3 is a side view of the waveguide of FIG. 2 taken in the directionof arrow 3;

FIG. 4 is a front view of the waveguide of FIG. 2 taken in the directionof arrow 4;

FIG. 5 is a top view of the waveguide of FIG. 2 taken in the directionof arrow 5 and depicting the magnetic field pattern in the top waveguidecompartment;

FIGS. 6 a, 6 b are similar views to FIG. 4 but depicting the orthogonalmagnetic field pattern in each compartment for LHCP and RHCP;

FIG. 7 a and FIG. 7 b depict graphs of return loss vs. frequency showingthe match of the right and left hand circular ports respectively for thewaveguide of FIG. 2;

FIG. 8 is a graph of signal insertion loss versus frequency for thewaveguide of FIG. 2;

FIG. 9 a is a graph of signal isolation (dB) vs. frequency depictingsignal isolation in the waveguide;

FIGS. 9 b and 9 c are graphs of signal cross-polar isolation (dB) forright-hand circular polarisation (RHCP) versus frequency on the LNBshown in FIG. 1 for right-hand circular polarisation (RCHP) and lefthand circular polarisation (LHCP) respectively;

FIG. 10 depicts a perspective and partly broken-away view of a waveguideof circular cross-section in accordance with an alternative embodimentof the invention;

FIGS. 11 a, b show a waveguide with a curved septum;

FIG. 11 c shows a plot of insertion loss and cross-polar isolationversus frequency for the waveguide of FIG. 11 a and FIG. 11 b;

FIG. 12 is a similar view to FIG. 3 but shows the rear wall of thewaveguide formed with a ground plane of a printed circuit board, and

FIGS. 13 a,b depict views similar to FIG. 4 of alternative probearrangement within the waveguide.

Reference is first made to FIG. 1 of the drawings which depicts a lownoise block (LNB) generally indicated by the reference numeral 20 whichhas a corrugated horn 22 coupled to a aluminium alloy (LM24) waveguide24 of square cross-section which is fabricated with an integral rearwall and integral cast alloy portion 26 which supports a printed circuitboard (PCB) 28. The PCB 28 carries two probes 30 a,b one of which (30 b)is shown, which pass through the rear wall 32 of the waveguide fordetecting a linearly polarised signal which is connected by the PCB totwo coaxial connector 34 (only one which is shown in the interest ofclarity), from where electrical signals are coupled to a set top box orreceiver.

The LNB 20 is particularly suited for the United States market wheresignals are transmitted using dual circular (left and right)polarisation over a frequency band OF 12.2 to 12.7 GHz. From FIG. 1 itwill be seen that the waveguide contains a stepped septum 36 whichseparates the waveguide into two equal compartments as will be laterdescribed, each compartment receiving a probe for detection of therespective polarised signal.

Reference is now made to FIGS. 2 a and 2 b of the drawings which depictthe waveguide 24 in more detail. The square waveguide 24 is separatedinto two waveguide compartments 38, 40 by the stepped septum 36. Thestepped septum 36, in conjunction with the surrounding waveguide walls,converts the left and right circularly polarised signals into linearlypolarised signals over the length of the septum such that by the timethe signals reach the septum portion in the vicinity of probes 30 a,bthe signals are linearly polarised for detection by the probes 30 a,b.In the embodiment shown, the aluminium alloy waveguide is approximately15.1 mm square; the septum is 44 mm in length, 1.5 mm thick and is alsoformed with a 1° draft angle per septum side to facilitate manufacture.

Reference is now also made to FIGS. 3 and 4 of the drawings. It will beseen that probes 30 a and 30 b are not straight; each probe 30 a, 30 bhas a first portion 41 a, 41 b which extends 7.3 mm into the respectivewaveguide compartments 38 and 40 in a direction parallel to the septum36 and main waveguide axis 42. The probes 30 a and 30 b then bend intoportions 44 a and 44 b which are 7.35 mm long and which terminate inprobe leading ends 46 a and 46 b disposed in proximity to the surface ofthe septum 36. In the embodiment shown, portions 44 a and 44 b areangled to portions 41 a and 41 b in planes 49, 51 in which the anglesare 120°; best seen in FIG. 3, and the probes 44 a, 44 b makesrespective angles of 20° with planes 48, 50 as seen in FIG. 4.

This probe design and orientation results in a cross-polar isolationvalue which exceeds the 25 dB signal isolation standard of the UnitedStates and as will be later seen, can approach or exceed 30 dB. Thepositioning of the free ends of the probes 46 a and 46 b in proximity tothe septum creates a sufficiently high capacitive coupling with theseptum to allow the probes to couple into the magnetic field.

This is best seen in FIG. 5 of the drawings which depictsdiagrammatically the magnetic field pattern in the top waveguidecompartment 38 of waveguide 20. It will be seen that probe portion 46 awithin the waveguide detects the magnetic field as shown and theresulting detected signal is fed by the probe 30 a to coaxial section56.

Reference is also made to FIGS. 6 a and 6 b of the drawings whichdepicts respective magnetic field pattern in each compartment 38 and 40for detection by the probes 30 a and 30 b. In FIG. 6 a the magneticfield is shown for the converted LHCP polarisation in compartment 38with field lines coming out of the paper (−) and entering the paper (+).It will be seen that there is minimal field in compartment 40 for thiscase. FIG. 6 b shows the field pattern for the converted RHCP with thefield lines being arranged in compartment 40 in the opposite directionto FIG. 6 a and minimal field shown in compartment 38.

Referring now to FIGS. 7 a and 7 b of the drawings, which depict thereturn loss (dB) showing the match of the right-hand circular port andleft-hand circular port, it will be seen that the match in both theleft-hand circular port and the right-hand circular port is greater than10 dB across the frequency band of interest which will allow anacceptable noise figure level to be achieved from the LNB.

FIG. 8 of the drawings depicts a graph of the insertion loss (dB) forthe waveguide in FIG. 2 for right-hand circular and left-hand circularpolarisations. It will be seen that the insertion loss across the bandof interest including connectors and feed is less than 1 dB.

Reference is also made to FIG. 9 a of the drawings which is a graph ofsignal isolation in the waveguide of FIG. 2 for right-hand circular(RCHP) and left-hand circular polarisations (LHCP). It will be seen fromFIG. 9 a that for both right-hand circular and left-hand circularpolarisation the isolation exceeds 25 dB across the band of interest,12.2-12.7 GHz. For right-hand circular polarisation the isolationexceeds 30 dB across the band of interest. Thus the performance of thewaveguide exceeds the 25 dB signal isolation requirement across thefrequency band of interest.

Reference is also made to FIGS. 9 b and 9 c which are graphs ofcross-polar isolation for the dual output LNB shown in FIG. 1 for boththe RHCP and LHCP. Each plot contains two isolation traces, one with theopposite output switched to LHCP and one with the opposite outputswitched to RHCP because the LNB is dual output. It will be appreciatedby those skilled in the art that the total isolation figure of an LNBdepends on many factors including, but not limited to the cross-polarisolation of the waveguide. FIG. 9 b depicts the cross-polarisation forthe RHCP and it will be seen that the isolation figure for the LNBexceeds 25 dB across the entire frequency band of interest andapproaches −30 dB at the upper end of the band for both the LHCP andRHCP signals. Similarly, it will be seen from FIG. 9 c that thecross-polar isolation for the LNB for LHCP exceeds 30 dB across thefrequency band of interest and it exceeds 35 dB at the upper end of thefrequency band.

Therefore, it will be understood that in the embodiment of the waveguidedescribed with reference to FIGS. 1 to 9, that this structure provides awaveguide and LNB which satisfies the 25 dB cross-polar signal isolationrequirement in a waveguide for receiving left and right hand circularpolarisation signals (LHCP and RCHP) with two probes across thefrequency band of interest 12.2 to 12.7 GHz.

FIG. 10 of the drawings depicts an alternative embodiment of a waveguidefor use in an LNB such as that shown in FIG. 1. In this embodiment thewaveguide 60 is circular in cross-section but the shape of the septum 36and probes 30 a, b, and the probe orientation is substantially identicalto that shown and described in relation to the first embodiment. Thisarrangement produces a similar performance to that of the squarecross-section waveguide 24.

It will also be appreciated that various modifications may be made tothe waveguide structures hereinbefore described without departing fromthe scope of the invention. In the structure described with reference toFIGS. 1 to 9 and the structure FIG. 10, it will be seen that the septumis stepped. However, septum does not require to be stepped and asmoothly curved septum could be used instead. Reference is made to FIG.11 a which depicts a waveguide 70 with a septum 72 which is non-steppedand which has a continuously curved edge 74. The septum is 1 mm thick,15 mm wide and the edge 74 is defined by the radius of a circle ofradius 46 mm as shown in FIG. 11 b.

FIG. 11 c shows the principal performance parameters of waveguide 70. Itwill be seen that the insertion loss over the frequency band of interestis minimal and the isolation loss over the same band of interest exceeds25 dB, also exceeding the U.S. specification requirement.

Reference is now made to FIG. 12 of the drawings which depicts analternative probe-mounting structure for the waveguide 24. In FIG. 12,it will be seen that the waveguide 24 has a rear wall 80 which is formedby the ground plane 82 of a printed circuit board 84. Probes 30 a and 30b pass through the ground plane and are coupled to tracks on the circuitboard 80. This construction has the advantage that the square orcircular waveguide is easier to manufacture and the ground plane of thecircuit board can be utilised as the rear wall of the waveguide.Performance figures for the structure shown in FIG. 12 are similar tothose for the arrangement described with reference to FIGS. 1 to 9.

Further modifications may be made to the embodiments hereinbeforedescribed without departing from the scope of the invention. Forexample, the angle between probe portions 41 and 44 does not require tobe 120° or an obtuse angle. It may be a right-angle or even an acuteangle. It will be appreciated, the angles between planes 49, 48 and 51,54 may be varied slightly with minimal degradation of performance. Theleading ends of the probe require to be located in proximity to thewaveguide wall or septum such that a relatively high capacitance iscreated to achieve satisfactory magnetic coupling to the probes. FIGS.13 a, 13 b depict alternative probe arrangements. FIG. 13 a shows theprobes reflected about plane 80 but still converging towards septum 36.FIG. 13 b shows the probe 30 a reflected about plane 81 and 30 breflected about plane 82 with respect to the orientation shown in FIG.4. The probes in FIG. 13 b diverge from the septum with the free ends ofthe probes disposed in proximity to the waveguide walls. Each of thesearrangements provide an isolation performance which meets the 25 dBisolation specification. Each probe may be disposed in its respectivecompartment such that it is reflected about one or both of the planesbisecting the compartment with the free end of the probe disposed inproximity to the waveguide or the septum surface to allow the probe tocapacitively couple into the magnetic field of the primary orfundamental waveguide mode in the waveguide compartment. It will beappreciated that this allows a total of sixteen possible arrangementsfor the probes. The leading end of each probe is located such that thematch exceeds 10 dB across the band of interest. Furthermore, the probesdo not require to be shaped as shown in the drawings. A straight or acurved probe is sufficient as long as the leading ends of the probes arelocated in proximity into the waveguide or septum such that thecapacitance coupling achieves the appropriate magnetic coupling of thesignal of the waveguide in order to meet the performance targets. Itwill be appreciated that a great many probe shapes may achieve thissolution.

It will also be appreciated that the waveguide may be diecast in alloysother than aluminium, for example, zinc alloy, MAZAC, or magnesium alloyAZ91D, as well as being diecast from the elements zinc, aluminium andmagnesium themselves. It will also be appreciated that the waveguidehereinbefore described with reference to the LNB may be used withdifferent types of waveguide horns and LNB structures which aredifferent to that shown in FIG. 1 for use in different jurisdictions.

It will be appreciated that the design is reciprocal and can be used togenerate LHCP and RHCP in a transmitter rather than receiving thesesignals in an LNB. This would occur by energising the probes in thecompartments to generate the appropriate fields in the waveguide.

It will be appreciated that the principal advantage of the inventionhereinbefore described is that a waveguide structure is provided whichmeets the U.S. isolation requirements across the full frequency range.

1. A dual probe waveguide structure for use in a LNB (low noise block)for receiving a left (L) and a right (R) circularly polarisedelectromagnetic radiation signal and for converting the circularlypolarised signals into linearly polarised signals, the waveguidestructure comprising a waveguide housing of a substantially symmetricalcross section, said waveguide housing having a front aperture and a rearwaveguide wall, wherein the waveguide wall is provided by a ground planeof a circuit board disposed at the end of the waveguide, a septumdisposed within the housing and coupled to the rear waveguide wall andthe housing to separate the waveguide into two waveguide compartments,one compartment for receiving and converting the left circularpolarisation signal into a first linearly polarised signal and the othercompartment for receiving and converting the right circular polarisedsignal into a second linearly polarised signal orthogonal to the firstlinearly polarised signal, a first probe extending into said firstwaveguide compartment from the rear waveguide wall and a second probeextending into the second rear waveguide compartment from the secondwaveguide wall the first and second probes each having a free end, theprobes being oriented and arranged such that the free ends of the probesare disposed in proximity to the waveguide wall or septum in eachrespective compartment such that the probes capacitively couple into themagnetic field of the primary or fundamental waveguide mode in thewaveguide compartment.
 2. A waveguide structure as claimed in claim 1wherein the waveguide housing is square in cross-section.
 3. A waveguidestructure as claimed in claim 1 wherein the waveguide housing iscircular in cross-section.
 4. A waveguide structure as claimed in claim1 wherein the septum is stepped.
 5. A waveguide structure as claimed inclaim 1 wherein the septum is non-stepped and has a curved edge.
 6. Awaveguide structure as claimed in claim 1 wherein the rear wall of thewaveguide is integral with the waveguide housing.
 7. A waveguidestructure as claimed in claim 1 wherein two probes are mounted in thecircuit board, one probe extending into a respective compartment.
 8. Awaveguide structure as claimed in claim 7 wherein the probes, arecircular in cross-section.
 9. A waveguide structure as claimed in claim7 wherein the probes are of any other suitable cross-section, such assquare, rectangular, hexagonal or triangular, which maximises thecoupling of the magnetic field from the compartment.
 10. A waveguidestructure as claimed in claim 1 wherein each of the probes has a firstportion which extends substantially parallel to the waveguide axis intothe respective waveguide compartment and a second portion coupled to thefirst portion at an obtuse angle, each second portion having its freeend disposed towards the septum and the leading end of the other probe.11. A waveguide structure as claimed in claim 10 wherein the free endsof the probes converge towards each other and towards the septum.
 12. Awaveguide structure as claimed in claim 10 wherein the free ends of theprobes diverge from the septum towards the waveguide wall.
 13. Awaveguide structure as claimed in claim 1 wherein the probes are locatedin respective compartments such as to be reflected about the plane ofthe septum.
 14. A waveguide structure as claimed in claim 1 wherein thewaveguide housing, rear wall and septum, are formed from a die-castmetal selected from aluminum, zinc, magnesium or alloys of theseelements such as MAZAC, a zinc alloy; LM24, an aluminum alloy, andAZ91D, a magnesium alloy.
 15. A waveguide structure as claimed in claim1 wherein the septum is substantially the same thickness from the rearwall to the stepped or curved edge of the septum, the septum having adraft angle about 1° per side to facilitate release of the waveguideafter being die-cast.